1. Field of Invention
The invention relates to the orientation of polymers and, more particularly, to orienting copolymer microdomains in thin films.
2. Description of Related Art
The creation of a regular surface pattern on the nanometer scale is important for many applications. For instance, periodic arrays constructed by optical microlithography are used as separation media in electrophoresis, as described by Volkmuth et al. in “DNA electrophoresis in microlithographic arrays,” Nature 358, 600 (1992). Island structures can be used for high density magnetic recording devices, as described by Chou et al., in “Single-domain magnetic pillar array of 35-nm diameter and 65-Gbtits/in2 density for ultra high density quantum magnetic storage,” J. Appl. Phys. 76, 6673 (1994). Periodic nanostructures can also be useful, for instance, in thin films as templates for lithography as described by Mansky et al., in “Monolayer films of diblock copolymer microdomains for nanolithographic applications,” J. Mater. Sci. 30, 1987 (1995), Park et al. in “Block copolymer lithography: periodic arrays of ˜1011 holes in 1 square centimeter,” Science 276, 1401 (1997), Thomas et al. in Macromolecules, 20, 2934 (1987), Lammertink et al. in Adv. Mater., 12, 98 (2000), and Li et al. in Appl. Phys. Lett., 76, 1689 (2000). By chemically removing one polymer from a thin film, patterns in the film may be transferred to a substrate either through reactive etching or by thermal evaporation of a component into the previously removed regions. Additionally, the domains can be used as a template for decoration with nanoparticles as described by Boontongkong et al. in Chem. Mater., 12, 1628 (2000), Fogg et al. in Macromolecules, 30, 8433 (1997) and Fink et al. in J. Lightwave Technol., 17, 1963 (1999).
One class of materials that has shown promise in forming such nanostructures includes block copolymers. Block copolymers generally include chemically distinct macromolecules covalently linked to form a single chain and due to their mutual repulsion, the dissimilar blocks tend to segregate into different domains whose shape, size and spacing are determined by the relative amount of the block components and their respective molecular weights. See, for example, Muthukumar et al. in “Competing interactions and levels of ordering in self-organizing polymeric materials,” Science 277, 1225 (1997), and Muthukumar et al. in “Competing interactions and levels of ordering in self-organizing polymeric materials,” Science, 277, 1225 (1997). As a result, the self-assembly features of block copolymers may be harnessed to produce structures on the nanoscopic length scale as described, for example, by Bates et al. in Annu. Rev.Phys. Chem., 41, 525 (1990); Fink, et al. in J. Lightwave Tech., 17, 1963 (1999), Urbas et al. in Adv. Mater., 12, 812 (2000), Li et al. in Appl. Phys. Lett, 76, 1689 (2000), and Park et al. in Science, 276, 1401 (1997).
Optimal utilization of nanoscopic patterns, however, requires spatial and orientational control of microdomains in a material. Indeed, the microdomains composed of the different blocks, having sizes of several tens of nanometers, typically nucleate randomly and grow into a polygranular texture, with periodic ordering maintained only over distances of about 50 lattice constants (i.e., a grain size of only 1-2 microns). A greater range of engineering applications demands control over the orientation and position of the microdomains. Thus, the development of processing techniques which create global orientation of the microdomains in block copolymer thin films is an important goal.
In crystalline materials, control of the solidification process is key to many technologies which rely on the features of the resultant microstructure for achieving optimum properties. For example, the directional solidification of a eutectic metal alloy can lead to rod or lamellar structures well aligned along to the growth direction. See, for example, Hashimoto et al. in “In Block Copolymers, Science and Technology,” D. J. Meier ed. (Harwood Academic Publ. London, p 63-108, 1983). In crystalline polymeric materials, orientation of crystallizable macromolecules has been achieved by mechanical forces as in fiber spinning and also by epitaxial crystallization onto substrates, as described by Swei et al. in “Encyclopedia of Polymer Science and Engineering,” (Wiley, N.Y., 6, 209, 1986), Wittmann et al. in “Epitaxial crystallization of polymers on organic and polymeric substrates,” Prog. Polym. Sci., 15, 909 (1990).
Although some success has been found in the formation of such regular surface patterns, existing techniques may in some cases be time consuming, difficult to control, and/or not allow the formation of patterns having a desired size, shape, orientation, periodicity, or other features. For example, control over domain orientation with block copolymer materials has been achieved using electric fields by Morkved et al. in “Local control of microdomain orientation in diblock copolymer thin films with electric fields,” Science 273, 931 (1996) and Mansky et al. in “Large-area domain alignment in block copolymer thin films using electric fields,” Macromolecules 31, 4399 (1998). Domain orientation and lateral spacing by deposition onto topographically patterned substrates has been shown by Fasolka et al. in “Observed substrate topography-mediated lateral patterning of diblock coplymer films,” Phys. Rev. Lett., 79, 3018 (1997), as well as by confining the block copolymer between neutral surfaces using random copolymer covered substrates and superstrates by Huang et al. in “Nanodomain control in copolymer thin films,” Nature, 395(6704), 757 (1998) and Huang et al. in “Using surface active random copolymers to control the domain orientation in diblock copolymer thin films,” Macromolecules,” 31, 7641 (1998). Other techniques have been used to induce alignment of the microdomains in block copolymers. See for example, Keller et al. in Nature, 225, 538 (1970); Hadziioannou et al. in Colloid Polym. Sci., 257, 136 (1979); Morrison et al. in Macromolecules, 23, 4200 (1990); Vigild et al. in Macromolecules, 31, 5702 (1998); Koppi et al. in J. Rheol., 38, 999 (1994); Pinheiro et al. in Macromolecules, 31, 4447 (1998); et al. in Polym. Eng. Sci., 36, 1414 (1997); Quiram et al. in Macromolecules, 31, 4891 (1998); Albalak et al. in J. J. Polym. Sci., Polym. Phys. Ed., 31, 37 (1993); et al. in Macromolecules, 32, 2075 (1999); Amundson et al. in Macromolecules, 27, 6559 (1994); Thurn-Albrecht et al. in Science, 290, 2126 (2000); Fasolka et al. in Macromolecules, 33, 5702 (2000); Huang et al. in Macromolecules, 31, 7641 (1998); Fasolka et al. in Phys. Rev. Lett., 79, 3018 (1997) and Rockford et al. in Phys. Rev. Lett., 82, 2602 (1999). Many of these techniques typically couple an externally applied field to some molecular and/or supermolecular feature in the polymer to achieve directional properties, such as transport, optical, and mechanical properties. According to these techniques, if an applied bias field (mechanical, electric, temperature, etc.) is present during the self-assembly process, preferential orientation can develop instead of random nucleation of microdomains. Control of film thickness is another way to vary the orientation of microdomains. Fasolka et al. in Macromolecules, 33, 5702 (2000) examined the phase behavior of a lamellar-forming block copolymer for film thicknesses less than the period of block copolymer on SiO2/Si substrates. They showed that either parallel or perpendicular ordering can be obtained depending on film thickness and interfacial energy between the substrate and the block copolymer. However, these approaches have not produced uniform periodic structures in which both microdomain components traverse across a thin film and present a chemically-patterned free surface.